The clinical problem: Approximately 3,500 people worldwide now undergo heart transplantation every year. Most heart transplant recipients are monitored for the development of organ rejection with a series of endomyocardial biopsies during the first twelve months after operation. This procedure is invasive, painful, and potentially hazardous. There is a clinical need for a test to provide the same information about heart transplant rejection as an endomyocardial biopsy, but without its invasiveness, pain, or hazards. A solution to the clinical problem: The Heartsbreath test is an intrinsically safe, painless, and non-invasive breath test for heart transplant rejection that employs volatile biomarkers of oxidative stress. In multicenter clinical studies, the Heartsbreath test was sensitive and specific for Grade 3 heart transplant rejection (now known as Grade 2R). The Food &Drug Administration (FDA) approved the Heartsbreath test for clinical use with a Humanitarian Device Exemption (HDE). The marketing problem: Even though FDA has approved the test for clinical use, the Heartsbreath test is not yet employed in clinical practice because it is not yet reimbursed by insurers. Medicare has not approved a National Coverage Determination (NCD) of the Heartsbreath test because of its HDE status. A solution to the marketing problem: Medicare now offers a pathway to insurance reimbursement of the Heartsbreath test through its program "National Coverage Determinations with Data Collection as a Condition of Coverage: Coverage with Evidence Development". This program offers a pathway to an NCD for the Heartsbreath test by developing and capturing additional patient data. Experimental plans: We will validate the accuracy of the Heartsbreath test as a predictor of Grade 2R (formerly known as Grade 3) heart transplant rejection, in order to fulfill Medicare NCD requirements. We will perform a cross-sectional multicenter study of 1,000 anonymized patients who have received a heart transplant during the preceding 12 months. We will perform a Heartsbreath test on these patients prior to an endomyocardial biopsy performed as part of their regular clinical care, and compare the results of the two tests. The long-term aims of the research are a. Scientific: To validate the accuracy of the Heartsbreath test as a breath test for heart transplant rejection b. Commercial: To obtain Medicare NCD for reimbursement of the Heartsbreath test, and market the test nationally and internationally. The importance of the research: In clinical practice, the Heartsbreath test could provide a completely safe and non-invasive ancillary test for heart transplant rejection that would reduce the pain and morbidity associated with endomyocardial biopsy, while also reducing the costs of clinical care PUBLIC HEALTH RELEVANCE: The Heartsbreath test is an FDA-approved breath test for heart transplant rejection that employs breath biomarkers of oxidative stress. It could provide a safe and non-invasive ancillary test for heart transplant rejection that would reduce the pain and morbidity associated with endomyocardial biopsy, and also reduce the costs of clinical care. We will validate the Heartsbreath test as a predictor of Grade 2R heart transplant rejection in order to fulfill Medicare requirements for reimbursement.